High urinary TWEAK is characteristic of lupus nephritis in systemic lupus erythematosus patients. (a) Thirty lupus nephritis (LN) patients have significantly higher urinary TNF-like weak inducer of apoptosis (uTWEAK) levels (corrected to creatinine) than 49 systemic lupus erythematosus (SLE) patients without LN (P < 0.001). (b) uTWEAK levels of 78 SLE patients (both LN patients and non-LN patients) correlate significantly with renal Systemic Lupus Erythematosus Disease Activity Index (rSLEDAI) scores (r = 0.388, P = 0.047). (c) Nonparametric receiver operating characteristic (ROC) curve for uTWEAK and the presence of LN in SLE patients; area under the curve = 0.724. (d) uTWEAK levels in 30 patients with biopsy-proven LN versus 11 SLE patients with a clinical diagnosis of LN not confirmed by renal biopsy (P = 0.941). Graphs represent median levels with the interquartile range. **P < 0.01.